Security Threat Model

---
name: security-threat-model
description: Model threats to a system with STRIDE — scope, actors, a text data flow diagram with trust boundaries, threats rated by likelihood and impact, and mitigations. Use when the user is designing or changing a system or feature and asks what could go wrong or how an attacker might abuse it. Not for current CVEs and attacker groups in the wild; see /security-threat-intel.
metadata:
  argument-hint: "[project] [--skip-checks]"
  workflow: security
  phase: 1
  artifact: 01-identify.md
---

# Threat Model

Model threats for "$ARGUMENTS" using structured threat modeling methodologies. Identify what can go wrong, assess each threat's severity, and propose mitigations.

**Arguments.** `$ARGUMENTS` may contain the project name and optional flags. Take the first token as `<project>` and treat `--skip-checks` as a flag; never include flags in file paths. All paths below use `<project>`. If `--skip-checks` is present, skip the prerequisite check and, **before writing any artifact**, append one line to `.singularskills/log.jsonl`: `{"ts":"<ISO timestamp>","skill":"security-threat-model","workflow":"security","phase":1,"project":"<project>","event":"skipped","artifact":"01-identify.md","note":"<reason or \"none given\">"}`.

## Prerequisites

**Part of a workflow.** This skill is phase 1 (Identify) of the NIST CSF workflow (`security`); cross-domain workflows run it too. First check `.singularskills/log.jsonl` for an open run for this project: a `run-started` line with `"project":"<project>"` and no later `run-finished` line for the same `workflow` and `project`. If one exists, the conductor is running you: continue here. Otherwise, if the user asked to start, run, continue or resume a process or workflow rather than this one step, do not continue here: invoke the `workflow` skill with the workflow the user asked for, or `security` when they named none, followed by `<project>`; it runs this phase in order, asks before it writes and records the run. Otherwise continue here: a later phase runs on its own, behind its gate.

Run this skill's gate before writing: `sh <this skill's folder>/scripts/gate.sh <project> <project root>`, where `<project root>` is the folder that contains `.singularskills/` (use `.` when you are already there). If it exits non-zero, stop and tell the user what it printed, unless `--skip-checks` was given (see Arguments). On a refusal, before you stop, append this line to `.singularskills/log.jsonl` (create the folder and file if missing): `{"ts":"<ISO timestamp>","skill":"security-threat-model","workflow":"security","phase":1,"project":"<project>","event":"refused","artifact":"01-identify.md","note":"<what the gate printed>"}`. Once the gate has passed, or under `--skip-checks` once you have appended the `skipped` line, append this line before you start work: `{"ts":"<ISO timestamp>","skill":"security-threat-model","workflow":"security","phase":1,"project":"<project>","event":"started","artifact":"01-identify.md","note":""}`.

Read `.singularskills/security/<project>/00-govern.md`. If this file does not exist, tell the user:

> Phase 0 (Govern) has not been completed for "<project>". Run a Govern skill first (e.g., `/security-security-policy <project>`), or use `--skip-checks` to bypass.

## Process

1. **Read context files:**
   - Read `references/threat-model-template.md` for methodology reference and output structure
   - Read `.singularskills/security/<project>/00-govern.md` for organizational context and risk appetite

2. **Describe the system being modeled:**
   - Define the system or component scope (application, service, infrastructure, data flow)
   - Identify key actors: end users, administrators, external services, automated processes
   - List technologies and protocols in use
   - Document assumptions about the environment (network segmentation, authentication mechanisms, hosting model)

3. **Draw data flow diagram (text-based):**
   - Identify processes, data stores, external entities, and data flows
   - Use text-based notation to represent the diagram (e.g., `[User] --> (Web App) --> [Database]`)
   - Mark trust boundaries between different security zones (public internet, DMZ, internal network, database tier)
   - Label each flow with the data type and protocol

4. **Apply STRIDE to each component:**
   - For each element in the data flow, systematically evaluate:
     - **Spoofing** — can an attacker impersonate a legitimate user or system?
     - **Tampering** — can data be modified in transit or at rest without detection?
     - **Repudiation** — can an actor deny performing an action without proof?
     - **Information Disclosure** — can sensitive data be exposed to unauthorized parties?
     - **Denial of Service** — can the component be rendered unavailable?
     - **Elevation of Privilege** — can an attacker gain higher access than authorized?
   - Document each identified threat with ID, description, affected component, and STRIDE category

5. **Rate each threat by likelihood and impact:**
   - **Likelihood** (1-5): based on attack complexity, required access, and known exploitation
   - **Impact** (1-5): based on data sensitivity, business criticality, and blast radius
   - **Risk score** = likelihood x impact
   - Categorize: Critical (20-25), High (12-19), Medium (6-11), Low (1-5)
   - Prioritize threats by risk score for mitigation planning

6. **Write the artifact** — append to `.singularskills/security/<project>/01-identify.md` under a `## Threat Model` section. Create the file with a `# Identify — <project>` heading if it does not exist; if a `## Threat Model` section already exists, replace only that section. The section contains:

   - **System Description** — scope, actors, technologies, and assumptions
   - **Data Flow Diagram** — text-based diagram with trust boundaries
   - **Threat Catalog** — all identified threats with STRIDE classification and risk score
   - **Threat Prioritization** — ranked list by risk score
   - **Proposed Mitigations** — recommended controls for each high and critical threat

## Output

The threat model appended to `.singularskills/security/<project>/01-identify.md`. Present a summary to the user highlighting:
- System scope and trust boundaries identified
- Total threats found by STRIDE category
- Top critical and high-risk threats
- Recommended mitigations for the highest-priority threats

**Record the run.** After you have written your section, append this line to `.singularskills/log.jsonl` (the `started` line of Prerequisites, with `"event":"finished"`): `{"ts":"<ISO timestamp>","skill":"security-threat-model","workflow":"security","phase":1,"project":"<project>","event":"finished","artifact":"01-identify.md","note":""}`.

This artifact is shared by every skill of phase 1 (Identify). The frontmatter is written once, by whoever creates the file: if you create it, open it with the block below (`skill` is the phase title, `started` the time of that first write; `gate` is `passed` when the gate ran clean and `skipped` under `--skip-checks`); if it already exists, leave the block alone except to update `finished` and to set `gate` to the worst value among the sections (`skipped` over `passed`).

```yaml
---
skill: Identify
workflow: security
phase: 1
project: <project>
started: <ISO timestamp>
finished: <ISO timestamp>
agent: <the agent you run in, or "unknown">
gate: <passed|skipped>
---
```

Open your `## Threat Model` section with one italic line, `*<skill name> · <ISO timestamp> · gate: <passed|skipped>*` (your skill name is `security-threat-model`).

End your section with `### Decisions` and `### Open questions` (one level under your `## Threat Model` heading, so the section stays one block), in this shape:

```markdown
### Decisions
- <what was chosen> — <why> — <what was rejected>

### Open questions
- <what the next phase or a reviewer must settle>
```

End the artifact with this paragraph, verbatim:

> **Not a penetration test or audit.** This artifact is generated analysis; validate findings against the live system and have material controls reviewed by a qualified security professional.